1. Field of the Invention
The present invention concerns a multifocal contact lens.
Multifocal contact lenses, which are especially intended for long-sighted persons, are designed so that they simultaneously form several images of one and the same object located in the field of vision. These various images correspond to different dioptric corrections, providing for good eyesight, both long-distance and short-distance eyesight, with the selection between the different images formed on the retina being made spontaneously by the cortex.
(The term "long-distance eyesight" refers herein to eyesight entailing accommodation to infinity or near infinity. The term "short-distance eyesight" refers herein to eyesight entailing maximum, accommodation to the subject or a neighbouring, accommodation (typically for reading) and the term "mid-distance eyesight" refers to eyesight corresponding to any accommodation located between these two extremes).
2. Description of the Prior Art
There are known lenses of this type (see, in particular, EP-A-0 232 191) comprising a spherical ring-shaped zone which provides long-sight correction and an aspherical central zone which provides short-sight and mid-sight correction, this central zone having a dome-shaped surface of revolution bulging out on the outer surface of the lens.
Since it has two zones, a spherical zone and an aspherical zone, a lens of this type exhibits, on the whole, dioptric power which varies from the center towards the periphery. However, it is noted in practice that while lenses of this type give good short-distance eyesight (through the central part of the aspherical central zone) and good long-distance eyesight (through the spherical ring-shaped zone), they are not very satisfactory for mid-distance eyesight. This is because of the transition in dioptric power between short-sight correction and long-sight correction.
Apart from the fact that it does not give the wearer satisfactory mid-distance eyesight, this drawback increases his eye fatigue because of the difficulty experienced by the cortex in selecting a satisfactory image of an object located at mid-distance.